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Personal Identity

Logic of Identity

(Text as at 26/11/2007 23:25:26)

*** THIS IS NOT THE LATEST VERSION OF THIS NOTE ***

Identity as a logical concept is rather uninteresting. There’s only one type of identity – that which satisfies Leibniz’s Law of the indiscernibility of identicals and is an equivalence relation. While rejecting them, I will still need to consider relative¹ identity and other so-called “identities”, such as vague² or indeterminate identity, contingent³ identity and occasional⁴ identity, for the light they shed on the issues at hand. Wrong answers to tough questions can be enlightening.

Note that Butler, Reid, Hume⁵ and some later philosophers’ support for the alleged distinction between the “loose and popular” and “strict and philosophical” senses of identity comes from confusing a thing with its stages⁶. The temporal stages of any continuant that changes over time (and the relational properties of all physical objects do) aren’t identical to one another, by Leibniz’s Law, yet the thing itself remains self-identical.

However, the concept of identity is useful for teasing out what it is for something to be what it is. It helps us spell out what the nature of an F is; there’s a connection between what it is to be an F and what it is for an F to (continue to) be. Identity comes with persistence criteria⁷, what it is to continue to be the same F. Also, we need to distinguish metaphysical questions from epistemological criteria of identity. It’s more important to answer the metaphysical question of what a thing is, and how it remains the same thing over time in the face of change (for which we need diachronic identity criteria, persistence criteria) than to answer the epistemological question of how we know what a thing is (for which we need mainly synchronic identity criteria).

I also need to consider Haecceity⁸ (primitive “Thisness”), the idea that what makes a thing the same thing over time is nothing to do with its persisting qualities.

Note also that the term “personal identity” can be used by philosophers, such as Parfit⁹, who don’t think that identity is properly involved (but merely psychological connectedness¹⁰), and by others such as Olson¹¹, who don’t think personhood is necessary for “our” continued existence.